1. Field of the Invention
This invention relates to the bending of glass sheets, and more particularly to a method of bending glass sheets in which there is an initial gravity bending step and a subsequent press bending step. Such a method is especially, but not solely, useful for bending automotive glass for subsequent laminating. The invention also relates to an apparatus for use in the method.
2. Summary of Related Art
Glass for the windows of vehicles is normally curved, the curvature being imparted to planar glass by a bending process. In one such bending process, planar glass sheets are placed upon female ring moulds and heated to the glass softening point. Each sheet bends ("sags") under its own weight until the periphery of the glass sheet is in contact with the ping mould. Such a bending technique is known as "sag" or gravity bending, and it has been developed over the years so as to bend glass which meets the demands of vehicle manufacturers.
For example, as mope deeply bent glass was required, the ring mould was modified by attaching the ends of the mould to the central portion by hinges, the hinged mould ends progressively closing as the glass softened and the bend proceeded. This avoided the tendency for the glass sheet to slide relative to the mould during the bend, thereby preventing scratching. Such a mould is commonly termed an articulated mould.
The gravity bending process has been found particularly suitable for the production of glass that is to be subsequently laminated by combining two sheets of glass with a sheet of interlayer material. The gravity bending process is capable of producing glass to a high optical quality, and it is also possible to bend two sheets of glass simultaneously, thereby producing a matched pair of glasses which give an excellent fit on laminating.
In recent years, developments in vehicle design have required glass of complex curvature, i.e. glass that is bent in two directions generally at right angles to each other. It is not possible to impart more than a very limited degree of complex curvature to a sheet of glass by gravity bending alone.
Furthermore, increased use of automated assembly by vehicle manufacturers demands that tighter dimensional tolerances be met by the glass. The shape of the periphery of the bent pane must be accurate, not only in terms of its two dimensional projection, but also in three dimensions, i.e. the angle of the glass adjacent the periphery must be correct. If this "angle of entry", as it is known to those skilled in the art, is not correct, the bent pane will not fit and seal satisfactorily in the receiving flange of the vehicle bodywork.
Moreover, the optical properties of the window depend on the shape of the central region of the glass which must therefore be accurately controlled in order that the required optical standards may be met.
These requirements, in conjunction with the trend to deeper, and more complex bends, can no longer be met by glass bent by the gravity bending technique alone. It is now regarded as necessary to complete the bending of such shapes by a subsequent press bending step. This step may only involve a limited part of the area of the bent pane, for instance the areas that, after installation in a vehicle body, will be adjacent the windscreen pillars of said body. In many current vehicle designs, such areas of the pane are required to be more deeply bent, and in this specification, any area of a pane which is required to be more deeply bent by means of a subsequent press bending step will be referred to as a deep bend portion.
An example of such a subsequent press bending step is given in European Patent Application number 89103298.9 in the name of Asahi Glass Company Ltd., and published as EP 338 216. This discloses an arrangement in which a pair of auxiliary male moulds press the deep bend portions of a pane against a ring mould. Either the hinged ends of an articulated mould on which the gravity bending has taken place are securely locked in position during the press bending step, or the glass is transferred to a non-articulated, i.e. Fixed, mould between the gravity bending step and the subsequent press-bending step. Such transfer is undesirable because the glass may lose shape, or temperature, or both in the course of the transfer. Furthermore, whether the press bending step is carried out on the articulated mould with the hinged ends locked in position, or after transfer to a fixed mould, it is necessary to extend the ends of the mould by `wide lands` to provide an adequate, correctly angled, alternative support for the deep bend portions, thereby reducing marking of the glass and attaining the required angle of entry during the press bending step. The term `wide land` is used to denote a ribbon-like strip of metal which is attached to the hinged end of the ring mould at an appropriate angle. It supports the glass sheet in place of the narrow upper edge of the ring mould end. However, use of such wide lands affects heat flow from the glass sheet, causing excessively high edge stress, and necessitating the taking of special measures to anneal the glass.
A disadvantage of this system as a whole is that every mould used for the pressing step needs to be reinforced to withstand the pressing force, equipped with wide lands, and if articulated, mould end locking devices. In a production furnace there may be in excess of 20 of these moulds, thereby resulting in a larger amount of fabrication work and expense in preparing a complete set of moulds.
For such reasons it is preferable not to use part of the ring mould, such as a hinged end, or an additional fixed ring mould, for supporting a deep bend portion of a sheet during the subsequent press bending step.